Cold Atomic Gas in the Small Magellanic Cloud

Abstract

In this project, we determine the mixture of thermal phases in the atomic medium of the Small Magellanic Cloud. The larger purpose is to study the difference in properties of the interstellar medium in a dwarf irregular galaxy compared with the Milky Way. The observations used the Australia Telescope Compact Array to measure absorption spectra of the 21 cm line through the SMC, for comparison with 21 cm emission spectra from a combination of data from the Parkes and Compact Array telescopes. Using the two together, we find the abundance of cool-phase gas to be less than 15% of the total H I. Thus, the cool-phase gas is very scarce, only about half as abundant relative to the total H I as in the solar neighborhood, a lower figure than that found in the LMC, M31, M33, or the solar neighborhood of the Milky Way. This is in qualitative agreement with the heating/cooling equilibrium calculations for conditions in a dwarf irregular galaxy. In addition, we find the temperature of the cool-phase clouds to be very low, typically 40 K or less. This is much less than the nominal diffuse cloud temperature in the Milky Way (50-100 K). This suggests that the H I may be more abundant in dense cold clouds in the SMC than in the Milky Way, where clouds of similar density and temperature are largely molecular.